This invention relates to improvements in fluid power control systems for hydraulically actuated, cooperating multiple load-engaging members normally mounted on lift trucks or other industrial vehicles. The multiple load-engaging members may be load-handling forks, clamp arms for load surfaces of curved, planar or other configurations, split clamp arms for handling multiple loads of different sizes simultaneously, layer picker clamp arms and their supporting booms, upenders, or other multiple load-engaging members movable cooperatively, but often differently, by linear or rotary hydraulic actuators. Differences in the respective cooperative movements of the respective multiple load-engaging members may include one or more differences in position, speed, acceleration, deceleration, and/or other variables. Although such differences are sometimes intended, they usually are unintended and cause the cooperating load-engaging members to become uncoordinated.
The respective movements of such cooperating mobile load-engaging members have conventionally been controlled either manually or automatically by fluid power valve assemblies which regulate respective flows of hydraulic fluid through parallel connections to separate hydraulic actuators which move each load-engaging member. Hydraulic flow divider/combiner valves are commonly used to try to achieve coordinated synchronous movements of such parallel-connected hydraulic actuators by attempting automatically to apportion respective hydraulic flows to and from the separate hydraulic actuators involved. However, such flow divider/combiner valves are capable of controlling only roughly approximate movements of separate hydraulic actuators, with the result that their presence in any hydraulic control system prevents highly accurate control of the actuators and allows accumulated errors. Other prior systems, which attempt to automatically correct unintended differences in the respective simultaneous movements of separate hydraulic actuators by monitoring their respective positions to provide feedback to respective hydraulic control valves, either variably regulate the separate control valves simultaneously, or completely block one of the valves until the correction has been completed, thereby substantially limiting the speed with which the actuators are able to complete their intended movements.